Conventionally, a semiconductor thin film such as polycrystalline silicon (poly-Si) is widely used for thin film transistors (TFTs: Thin Film Transistors) or solar cells. In particular, poly-SiTFTs are characterized by their high carrier mobility, and capability of being prepared on a transparent insulating substrate such as a glass substrate. Taking advantage of such characteristics, the poly-SiTFTs are widely used, e.g., as a switching element that structures a pixel circuit such as a liquid crystal display apparatus, a liquid crystal projector, or an organic EL display apparatus; or as a circuit element of a driver for liquid crystal drive.
One of the methods for preparing high-performance TFTs on a glass substrate is a manufacturing method generally referred to as the “high-temperature process”. Among the TFT manufacturing processes, the process that uses high temperatures, in which the maximum temperature during the processing steps is approximately 1000° C., is generally referred to as the “high-temperature process”. The characteristics of the high-temperature process lies in that: polycrystalline silicon of relatively high quality can be deposited through solid phase epitaxy of silicon; a high quality gate insulating layer can be obtained through thermal oxidation of silicon; and an interface between clean polycrystalline silicon and the gate insulating layer can be formed. With the high-temperature process, thanks to those characteristics, high-performance TFTs with high mobility and high reliability can stably be manufactured.
On the other hand, since the high-temperature process is the process of performing crystallization of a silicon film through solid phase epitaxy, heat treatment under a temperature of approximately 600° C. for a long time of approximately 48 hours is required. This is an enormously long step, and involves the issue that it essentially requires great number of heat treat furnaces in order to improve the throughput of the step, which makes it difficult to achieve a reduction in costs. In addition, since it is inevitable to use silica glass as the insulating substrate which is highly heat resistant, the cost of the substrate is high. Therefore, it is regarded that it is not suitable for increasing the area.
On the other hand, the technique for reducing the maximum temperature during the step to prepare poly-SiTFTs on a cost-effective and large-area glass substrate is the technique referred to as the “low-temperature process”. Among the TFT manufacturing processes, the process of manufacturing poly-SiTFTs on a relatively cost-effective heat-resistant glass substrate in the temperature environment in which the maximum temperature is approximately 600° C. or less is generally referred to as the “low-temperature process”. What is widely used in the low-temperature process is the laser crystallization technique in which crystallization of a silicon film is performed using a pulsed laser whose oscillation time is very short. The laser crystallization is the technique that exploits the nature of a molten silicon thin film crystallizing in the process of solidifying, after high output pulsed laser light is emitted to the silicon thin film on a substrate to instantaneously melt the silicon thin film.
However, the laser crystallization technique involves several significant issues. One of them is a large amount of trap levels locally present inside a polysilicon film formed through the laser crystallization technique. Because of the presence of the trap levels, the carriers that should originally move in the active layer by application of voltage are trapped and cannot contribute to electrical conduction. This has disadvantageous effects such as a reduction in mobility of TFTs and an increase in the threshold voltage. Another issue is that the limitation on the laser output limits the size of the glass substrate. In order to improve the throughput of the laser crystallization step, it is necessary to increase the area that can be crystallized at once. However, since the current laser output is limited, in the case where the crystallization technique is adopted for a large-size substrate, e.g., of the seventh generation (1800 mm×2100 mm), it requires a long time to crystallize a piece of substrate.
Further, in the laser crystallization technique, generally a linearly shaped laser is used. Scanning the laser, crystallization is achieved. Since the linear beam is limited in terms of laser output, it is shorter than the width of the substrate. Therefore, in order to allow the entire surface of the substrate to be crystallized, the scanning by the laser must be performed for several times. This results in the juncture region of the linear beams in the substrate, which is scanned twice. This region is largely different in crystallinity from the region where the crystallization is achieved by one-time scanning. Therefore, the element characteristic largely differs between the regions. This becomes a major factor of variations among the devices. Finally, since the laser crystallization apparatus has complicated device structure and the cost of the consumable parts is high, there is an issue that the apparatus cost and the running cost are high. Thus, the TFTs using a polysilicon film crystallized by the laser crystallization apparatus become the elements whose manufacturing cost is high.
In order to remove the issues such as the limitation on the size of the substrate and the high apparatus cost, the crystallization technique referred to as the “thermal plasma thermal jet crystallization method” is studied (e.g., see Non-patent Literature 1). This technique is briefly described in the following. When a tungsten (W) cathode and a water-cooled copper (Cu) anode are opposed to each other and a DC voltage is applied, arc discharge occurs between the electrodes. By allowing the argon gas to flow between the electrodes under the atmospheric pressure, thermal plasma is jetted out from the jet hole opening at the copper anode. The thermal plasma is thermal equilibrium plasma, which is an ultrahigh temperature heat source in which the temperatures of ions, electrons, and neutral atoms are substantially equal to one another, each being approximately 10000 K. Therefore, the thermal plasma can easily heat any heating target object to high temperatures. By the substrate on which an a-Si film is deposited being scanned at high speed on the thermal plasma front surface of the ultrahigh temperature, the a-Si film can be crystallized.
As described above, since the device structure is very simple and what is performed is the crystallization process under the atmospheric pressure, it is not necessary to cover the apparatus by an expensive member such as a chamber, and an extremely low apparatus cost can be expected. Further, since the utility required for crystallization is argon gas, electric power, and cooling water, it is the cost-effective crystallization technique in terms of the running cost also.
FIG. 16 is a schematic view for describing a semiconductor film crystallization method which uses this thermal plasma.
In FIG. 16, a thermal plasma generating apparatus 31 is structured to include a cathode 32, and an anode 33 disposed to be opposed to the cathode 32 by a prescribed distance. The cathode 32 is made of a conductor such as tungsten, for example. The anode 33 is made of a conductor such as copper, for example. Further, the anode 33 is formed to be hollow, such that water is allowed to pass through the hollow portion for cooling. Further, the anode 33 is provided with a jet hole (nozzle) 34. When a direct current (DC) voltage is applied between the cathode 32 and the anode 33, arc discharge is generated between the opposite electrodes. In this state, by allowing gas such as argon gas to flow between the cathode 32 and the anode 33 under the atmospheric pressure, the thermal plasma 35 can be jetted out from the jet hole 34. Here, the “thermal plasma” is the thermal equilibrium plasma, which is an ultrahigh temperature heat source in which temperatures of ions, electrons, and neutral atoms are substantially equal to one another, each being approximately 10000 K.
Such thermal plasma can be used for heat treatment for crystallization of a semiconductor film. Specifically, a semiconductor film 37 (e.g., an amorphous silicon film) is previously formed on a substrate 36, and thermal plasma (thermal plasma jet) 35 is blown in the semiconductor film 37. At this time, the thermal plasma 35 is blown in the semiconductor film 37 while being relatively shifted along the first axis (the right-left direction in the example shown in the figure) which is parallel to the surface of the semiconductor film 37. That is, the thermal plasma 35 is blown in the semiconductor film 37 while scanning in the first axis direction. As used herein, “to relatively shift” refers to relatively shift the semiconductor film 37 (and the substrate 23 supporting the same) and the thermal plasma 35, and includes both the case where only one of them is shifted and both of them are shifted. Such scanning of the thermal plasma 35 heats the semiconductor film 37 by high temperatures of the thermal plasma 35, to provide a crystallized semiconductor film 38 (a polysilicon film in the present example) (e.g., see Patent Literature 1).
FIG. 17 is a conceptual view that shows the relationship between the depth from the topmost surface and the temperature. As shown in FIG. 17, by shifting the thermal plasma 35 at a high speed on the substrate 36, only the proximity of the surface of the substrate 36 can be processed at high temperatures. After the thermal plasma 35 has passed, the heated region is quickly cooled and, therefore, the proximity of the surface achieves high temperature just for a short time.
Generally, such thermal plasma is generated at a dot-like region. The thermal plasma is maintained by thermionic emission from the cathode 32. Since the thermionic emission becomes more active at the position where the plasma density is high, the positive feedback is obtained, and the plasma density becomes even higher. That is, arc discharge occurs as being focused on one point of the cathode, and hence the thermal plasma is generated at the dot-like region.
In the case where a plate-like base material is desired to be processed evenly, such as crystallization of a semiconductor film, it is necessary to scan the dot-like thermal plasma over the entire base material. On the other hand, for the purpose of structuring the process with which the base material can be processed with reduced number of times of performing scanning and reduced time, it is effective to widen the emission area of the thermal plasma. Therefore, techniques for generating thermal plasma over a large area have long been considered.
For example, what is disclosed is a method for widening a plasma jet, in which width-widening gas for widening the width of the plasma jet is jetted simultaneously from each of two places in the direction crossing the center axis of the external nozzle to a plasma jet being jetted out from the external nozzle of the plasma torch (e.g., see Patent Literature 2). Alternatively, there is a disclosure of a method in which a plasma nozzle is provided, the plasma nozzle being characterized in that the opening portion of a nozzle passage is tilted by a prescribed angle relative to the axial center of the nozzle passage. A casing structuring the nozzle passage, or a part of the casing is rotated at a high speed about the longitudinal axial core, and the plasma nozzle is shifted to pass along a workpiece (e.g., see Patent Literature 3). Further, there is a disclosure in which a rotary head having at least one eccentrically arranged plasma nozzle is provided (e.g., see Patent Literature 4).
It is to be noted that, though it is not directed to process a large area in a short time, as a welding method using thermal plasma, a high-speed gas shielded arc welding is disclosed. The method is characterized in that a band-like electrode is used, and welding is carried out such that the width direction of the electrode is aligned with the welding line direction (e.g., see Patent Literature 5).
Further, there is a disclosure of an inductively coupled plasma torch forming a linear elongated shape, which employs a flat rectangular parallelepiped-shaped insulating material (e.g., see Patent Literature 6).
It is to be noted that, there is a disclosure of a method of generating elongated linear plasma using an elongated electrode (e.g., see Patent Literature 7). Though it is described that it generates thermal plasma, it generates low-temperature plasma and is not suitably structured for heat treatment. Provided that thermal plasma is generated, since it is the capacitive coupling type using an electrode, it is considered that arc discharge would be focused on one place to make it difficult to generate thermal plasma being uniform in the longer direction. On the other hand, as a low-temperature plasma processing apparatus, it is an apparatus that is capable of performing plasma processing such as etching or deposition by turning etching gas or CVD (Chemical Vapor Deposition)-use gas into plasma.